The origin of intramitochondrial pyridine nucleotides will be studied by 1) incubating rat liver mitochondria with NAD ion labeled with 14C in the adenine moiety and with 3H in the nicotinamide reisolating and washing the mitochondria and determining the 14C/3H ratio of the NAD contained in the matrix to determine if extramitochondrial NAD is transported intact across the inner membrane and 2) studying the nature of the reaction(s) catalyzed by the enzyme(s) responsible for the observed incorporation of 14C-nicotinamide into 14C-NMN and 14C-NAD in rat liver mitochondria.